* [[zhang_undistort]] is a tool distributed with [[hugin]] that uses [[CamChecker]] parameters to actually correct distortion.

* [[zhang_undistort]] is a tool distributed with [[hugin]] that uses [[CamChecker]] parameters to actually correct distortion.

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[[Category:Glossary]]

Revision as of 12:12, 11 November 2005

Barrel distortion is the opposite of pincushion distortion, it is a radial image distortion
introduced by the lens itself.

A photograph showing exaggerated barrel distortion

Panorama Tools a, b & c parameters

The Panorama Tools a, b & c parameters correspond to a third-order
radial polynomial describing radial lens distortion. It is never exact,
but can give a pretty good approximation to the real behaviour of a
given lens. The parameters are used by various tools to correct both
barrel distortion and pincushion distortion.

There are a number of ways to determine the a, b, c and fov
parameters for a particular lens/camera combination:

Taking two or more overlapping photographs and selecting lots of normal control points, then optimising roll, pitch, yaw, fov, a, b & c. This technique works with any output projection format (note that to get a really accurate measure of the field of view, you have to take a full 360 degree panorama).

Using a tool such as PTLens or clens to read the JPEGEXIF data and correct the image automatically by looking up the lens in an existing database.

The fov, a, b & c parameters are fairly consistent between shots
with the same camera/lens combination so you can use them again and
again. Though they can vary slightly depending on the
focus distance, so many panographers tend to recalculate
them for every stitch as part of their normal workflow.

The a & c parameters control distortion at the centre and edges of
the picture so are not very important, you can normally just zero
them and use the b parameter by itself.